In general, with the development of a very high speed, large capacity, long haul optical transmission system required in the optical Internet and large capacity optical transmission systems, a transmission rate of 7.5 Thz can be obtained using optical amplification technology with a wavelength bandwidth of 60 nm based on a 10 Gbit/s optical transmission system being currently commercialized.
However, 40-Gbit/s or beyond per channel transmission technology is required for the scheme of enabling very high speed transmission while reducing the number of channels because an effective transmission wavelength becomes narrower than the above-described wavelength bandwidth of 60 nm when guard wavelength intervals are taken into consideration in terms of cross talk between WDM channels.
Meanwhile, when the transmission rate per channel of an optical signal increases from 10 Gbit/s to 40 Gbit/s or beyond, signal distortion in an optical link is increased four or more times as much as at 10 Gbit/s due to the increase of required OSNR, chromatic dispersion, polarization mode dispersion and a non-linearity effect.
Of the above-described causes of signal distortion, the chromatic dispersion is increased 16 or more times as much as at 10 Gbit/s, so that a more accurate method is required to compensate for the chromatic dispersion for all the channels in broadband WDM transmission. Furthermore, signal distortion attributable to the polarization mode dispersion is increased four or more times as much as at 10 Gbit/s, so that an active polarization mode dispersion device is required to compensate for such signal distortion.
The increase of signal distortion restricts the transmission distance of an existing optical transmission system, and acts as a cause to change the configuration of an existing optical network.
In the meantime, a Non-Return-to-Zero (NRZ) signal scheme is advantageous in that the manufacturing costs thereof can be reduced because it allows the construction of a transmitter to be simplified, but is weak to signal distortion attributable to chromatic dispersion, polarization mode dispersion and a non-linearity phenomenon of the optical link. Accordingly, a Return-to-Zero (RZ) signal scheme is preferred.
Such a RZ signal scheme is advantageous in that receiver sensitivity at the receiver is superior, it is convenient to extract a clock signal, and signal distortion attributable to the non-linearity phenomenon is small, but is disadvantageous in that it is weak to chromatic dispersion because its optical spectrum bandwidth is wide.
Accordingly, the results of researches for improving transmission characteristics while reducing the spectrum bandwidth of an optical signal using a CS-RZ scheme have been reported. A CS-RZ signal is characterized in that it is robust to the non-linearity phenomenon of an optical fiber, can be transmitted over a long haul, and allows more channels in an available wavelength region to be utilized because the optical spectrum bandwidth thereof is narrower than that of the conventional RZ signal.
In the meanwhile, a conventional optical transmitter for generating a CS-RZ signal utilizes two external modulators, the first one of which is utilized to optically modulate NRZ data, and the second one of which is utilized to generate carrier-suppressed pulses. In this case, a RF signal input to the second external modulator is a clock signal having half of a data transmission rate and voltage two times of the first external modulator, with a bias voltage to be placed at the null point of the transfer function of the second external modulator. Thereby generated optical clock signal is mixed with a NRZ optical signal at an optical domain, thus producing a CS-RZ signal.
The reference papers related to conventional optical transmitters for generating CS-RZ signals are as follows:
1. Yutaka MIYAMOTO, Kazushige YONENAGA, Akira HIRANO, Masahito TOMIZAWA, N×40-Gbit/s DWDM Transport System Using Novel Return-to-Zero Formats with Modulation Bandwidth Reduction, IEICE Transactions on Communications, February 2002, Vol. E85-B, No. 2, pp. 374-385
2. Kiyoshi FUKUCHI, Kayato SEKIYA, Risato OHHIRA, Yutaka YANO, Takashi ONO, 1.6-Tb/s (40×40 Gb/s) Dense WDM Transmission Experiment Over 480 km (6×80 km) Using Carrier-Suppressed Return-to-Zero Format, IEICE Transactions on Communications, February 2002, Vol. E85-B, No. 2, pp. 403-409
3. Vassilieva, O.; Hoshida, T.; Choudhary, S.; Castanon, G.; Kuwahara, H., Numerical comparison of NRZ, CS-RZ and IM-DPSK formats in 43 Gbit/s WDM transmission, Lasers and Electro-Optics Society, 2001. LEOS 2001. The 14th Annual Meeting of the IEEE, Volume 2, 2001, pp. 673˜674
In the meantime, the CS-RZ signal is characterized in that it is robust to the non-linearity phenomenon of an optical fiber, such as Stimulated Brillouin Scattering (SBS), Self Phase Modulation (SPM) and Cross Phase Modulation (XPM).
However, the CS-RZ signal is characterized in that it is relatively weak to optical fiber dispersion, so that it is problematic in that the weakness makes the design and management of an optical link difficult. Furthermore, as shown in FIG. 1, a conventional method employs two external modulators for generating a RZ signal and modulating data to generate a CS-RZ signal. Accordingly, a problem arises in that the use of the two modulators increase the costs of an optical transmitter because the external modulators are most expensive in the construction of the optical transmitter.